This invention relates to a magnetic sensor for detecting a magnetic field strength of, for example, the earth""s magnetism, a magnetic signal, a magnetic field caused by a current, and so on, and in particular, to a magnetic sensor using a soft magnetic metallic element having an impedance that varies by exposure to the magnetic field.
A soft magnetic element, which has a high magnetic permeability, is know to have skin effect. When an AC voltage having a relatively high frequency on the order of MHz is applied to the soft magnetic metallic element, a ski depth due to the skin effect is determined by, and is proportional to, a reciprocal of a square root of its magnetic permeability. In addition, it is also known that a magnetic permeability of a magnetic material is apparently changed by a magnetic field in which the magnetic material is exposed. This means that an impedance of the soft magnetic metallic element changes by a magnetic field strength in which the soft magnetic metallic element is exposed. Therefore, it is possible to detect an external magnetic field by applying an AC voltage to the soft magnetic element and detecting an impedance change of the soft magnetic metallic element.
A known magnetic sensor of a type using the soft magnetic metallic sensor includes a magnetic sensor module which comprises an insulator substrate and an impedance bridge formed on the insulator substrate. The impedance bridge circuit comprises a pair of soft ma in metallic film strips and a pair of non-magnetic conductive film strips formed on the insulator substrate. The pair of soft magnetic metallic film strips are disposed in opposite arms of the impedance bridge circuit and the pair of non-magnetic conductive film strips are also disposed in the other opposite arms of the impedance bridge circuit. Thus, the impedance bridge circuit has four connection points connecting between adjacent arms. A high frequency oscillator is connected across two opposite ones of the four connection points for driving the magnetic sensor. And a voltage meter is connected across the other two opposite ones of the four connection points for detecting the external magnetic field as a voltage.
The impedance of the soft magnetic metallic film strips and the non-magnetic conductor strips are prepared so that the impedance bridge circuit is balanced with a zero volt of the voltage meter in a condition when the magnetic sensor is not exposed in any external magnetic field. When the magnetic sensor or the magnetic sensor module is exposed in an external magnetic field, the impedance of the soft magnetic metallic film strips changes and the voltage meter indicates a detection voltage. Thus, the external magnetic field is detected.
The magnetic sensor of this type has a magnetic sensitivity varying with the voltage level of the high frequency driving voltage and also by the magnetic level of the magnetic field applied thereto.
In order to increase the magnetic sensitivity, it is known in the prior art that a bias coil wound on a ring bobbin is used around the sensor module for magnetically biasing the sensor module. A bias DC current is applied to the bias coil to induce a biasing magnetic field which is applied to the sensor module. The DC current usually has a constant level so as to apply to the sensor module the magnetic bias of a fixed strength value.
In order to improve linearity of the detection output or to increase the magnetic sensitivity the bias DC current is controlled by a negative feed-back loop or a positive feed-back loop, respectively. Thus, a variable magnetic biasing is carried out.
However, the use of the bias coil wound on a ring bobbin results in a large size of the magnetic sensor and a high current consumption. For example, the coil has a diameter of 10 mm and the bias DC current is about 20mA.
In order to reduce noise, it is desired that the frequency of the driving AC voltage is high such as an order of MHz to 10 MHz. On the other hand, in order to make the magnetic sensor in small size, the soft magnetic metallic film strips are desired to have a reduced width and a reduced thickness. However, the frequency of the driving AC voltage is determined by the thickness of the strip. Therefore, reduction of thickness is not desired from a viewpoint that it results to an excessive high frequency of the driving AC voltage. While, the reduction of the width size causes increase of an area of a so called returning magnetic domain in the soft magnetic metallic film strip, which results in degradation of the magnetic sensitivity. Although the area of the returning magnetic domain is decreased by reduction of the thickness of the strip, it should be avoided from the viewpoint as described above.
Therefore, it is an object of this invention to provide a magnetic sensor using the soft magnetic metallic element having an AC impedance varying with a magnetic field strength applied thereto, which is small in size and improved in magnetic sensitivity.
It is another object to provide the magnetic sensor which has a constant or fixed magnetic bias structure without use of electric current therefor.
It is still another object of this invention to provide the magnetic sensor having a variable magnetic bias structure small in size.
It is yet another object of this invention to provide the magnetic sensor wherein an external magnetic field to be detected is effectively converged onto the soft magnetic metallic elements.
It is another object of this invention to provide the magnetic sensor where the soft magnetic element is a film strip having a reduced width size without increase of the area of the returning magnetic domain and without change of the frequency of the driving AC voltage.
According to this invention, a magnetic sensor is obtained which comprises a soft magnetic metallic element having impedance varying by exposure to an external magnetic field to be sensed, wherein said soft magnetic metallic element is formed in a zigzag shape.
In an aspect of this invention, the soft magnetic metallic element comprises a plurality of soft magnetic metallic strips equi-spacedly arranged in parallel with each other, and non-magnetic conductor pieces connecting adjacent ones of the soft magnetic metallic strips, respectively, to form a series connection of the soft magnetic metallic strips and the non-conductor metallic pieces alternately connected in the zigzag shape.
In an embodiment of this invention, the magnetic sensor comprises an impedance bridge circuit, the impedance bridge circuit comprising: first and second non-magnetic electro-conductive elements extending in a first direction and arranged in parallel with each other; first and second soft magnetic metallic elements extending in a second direction generally perpendicular to the first direction and arranged in parallel with each other to connect the first and second non-magnetic electro-conductive elements to each other at opposite ends, respectively, so that the first and second non-magnetic elements and the first and second soft magnetic elements form a rectangle having four connection points at the opposite ends, each of the first and second soft magnetic metallic elements being formed in the zigzag shape; two input terminals disposed at two diametrically opposite ones of the four connection points for applying an AC input voltage for driving the sensor; and two output terminals disposed at the other diametrically opposite ones of the four connection points for taking out an output signal.
In a preferred embodiment, each of the first and second soft magnetic metallic elements comprises a plurality of soft magnetic metallic strips extending in a third direction and equi-spacedly arranged in parallel with each other, and non-magnetic conductor pieces connecting adjacent ones of the magnetic metallic strips, respectively, to form a series connection of the soft magnetic metallic strips and the non-conductor metallic pieces alternately connected in the zigzag shape.
In another preferred embodiment, the magnetic sensor further comprises an insulator substrate, wherein each of the first and second non-magnetic electro-conductive elements, each of the soft magnetic metallic strips, and the non-magnetic metallic pieces are thin films formed on the insulator substrate.
The magnetic sensor may comprise a hard magnetic film formed on the insulator substrate, and a first interlayer film formed on the hard magnetic film, the thin films of each of the first and second non-magnetic electro-conductive elements, each of the soft magnetic metallic strips, and the non-magnetic metallic pieces being formed on the first interlayer insulator film.
The magnetic sensor may comprise a bias conductor film disposed at at least one level of the upper and the lower of the first and second soft magnetic metallic elements through an insulator layer for applying a DC bias magnetic field to the first and second soft magnetic metallic elements, the bias conductor film having opposite end terminals for receiving a DC current for exciting the DC bias magnetic field.
The magnetic sensor may comprise a protective top insulator layer having first through sixth apertures, the two input terminals being formed as first and second electrode pads exposed in the first and second apertures, the two output terminals being formed as third and fourth electrode pads exposed in the third and fourth apertures, the opposite end terminals being formed as fifth and sixth electrode pads exposed in the fifth and sixth apertures.
The magnetic sensor may comprise a second interlayer insulator film formed on the first and second non-magnetic conductive elements, the bias conductor film being a coil conductor film in a spiral shape formed on the second interlayer insulator film.
The magnetic sensor may comprises a protective top insulator layer having first through sixth apertures, the two input terminals being formed as first and second electrode pads exposed in the first and second apertures, the two output terminals being formed as third and fourth electrode pads exposed in the third and fourth apertures, the opposite end terminals being formed as fifth and sixth electrode pads exposed in the fifth and sixth apertures.
The magnetic sensor may comprises a second interlayer insulator film formed on the first and second non-magnetic conductive elements, the bias conductor film being a coil conductor film in a spiral shape formed on the second interlayer insulator film.
The coil conductor film may comprise a one-turn spiral pattern. Alternatively, the coil conductor film may comprise two one-turn spiral patterns having inner ends and outer ends, the two one-turn spiral patterns being connected to each other at their outer ends, the inner ends serving as the terminal ends.
The second direction is preferably same as the third direction. Two magnetic members with a high magnetic permeability are disposed at both outer sides of the first and second soft magnetic metallic elements in the third direction for converging fluxes of the external magnetic field onto the soft magnetic metallic elements.
In another aspect, each of the soft magnetic metallic strips has a multi-layer structure comprising soft magnetic metallic layers and at least one non-magnetic layer interposing between two of the soft magnetic metallic layers.